1. Field of the Invention
This invention relates to metallurgical apparatus, and more particularly to continuous steel strip annealing and coating apparatus.
2. Description of the Prior Art
In the continuous hot dip coating of metal strip, for example, the hot dip galvanizing or galvannealing of steel strip, it is conventional practice to continuously clean and anneal the cold rolled steel strip and pass it directly from the annealing furnace into a pot of molten coating metal through a chute containing a controlled or reducing atmosphere while the strip is still at or near the temperature of the molten coating metal. One continuous annealing furnace suitable for use in such an operation is shown in U.S. Pat. No. 3,622,140 and includes a cleaning zone for removing rolling oil and other contaminants from the strip surface and a plurality of heating zones wherein the strip is successively preheated, soaked, slow cooled, fast cooled, and held at the various temperatures for the times which make up the desired annealing cycle.
The annealed strip passes from the furnace into the molten coating metal bath where it is guided around a submerged sink roll then upwardly past a pair of guide or deflector rolls before emerging and passing between a pair of opposed air knives or coating thickness control nozzles where excess liquid coating material is removed as shown, for example, in U.S. Pat. No. 3,841,557. The coated strip may be contacted by a stabilizing roll, or a pair of such rolls, immediately above the air knives, but generally is not contacted as it passes vertically from the coating pot through a cooling zone for a distance sufficient to permit the liquid metal coating to solidify. In a galvannealing operation, the strip with the liquid metal coating adhering thereto passes into a second heating furnace such as the induction heating furnace shown in U.S. Pat. No. 4,761,530 where the temperature is quickly increased to a level to permit alloying of the coating metal with iron from the steel substrate before passing into the cooling zone.
At the top of the cooling zone, which may be up to 100 feet or more above the air knives, the coated strip passes over a cooled top guide roll from which it is withdrawn for further cooling and processing. The unsupported length of strip moving between the deflector roll in the coating metal bath (or the stabilizing rolls, if used), and the top guide roll tends to undulate or whip in a direction substantially perpendicular to its opposed surfaces and such movement (hereinafter sometimes referred to as vibrations) can be so severe as to cause defects in the coating. For example, movement of the strip toward one of the opposed air knives will reduce the coating thickness on that side and simultaneously increase the thickness on the opposing side, and such thickness variations depend on the extent of such movement. Further, rapid or excessive vibration of the strip as it emerges from the molten metal bath may result in spatter which can adhere to the coating thickness control nozzles and adversely effect their operation. This problem has become more critical in recent years with the increased use of hot dip coated steel strip in products such as automobiles having painted surfaces where even very minor variations in the metal coating can produce unacceptable blemishes in the finished surface. Further, in recent years the cost of zinc and zinc alloy coating metals used in hot dip galvanizing has greatly increased so that accurate control of the coating thickness can produce substantial savings.
Vibrations and undulations of the coated strip moving through a galvannealing furnace or through the cooling chamber also impose limitations on the operation. For example, excessive movement of the strip can result in contact between the strip and in coils or other structure of induction galvanneal furnaces and may result in uneven alloying of the coating. Also, in the cooling chamber, cooling fluid is directed onto the surface of the strip and excessive movement can result in uneven cooling or even contact between the strip and fixed structure in the cooling chamber.
In order to minimize vibration, it is desired to maintain the coated strip under relatively high tensile loads between the annealing furnace and the top guide roll. It has been found, however, that variations in the tensile load can induce or aggravate whipping, and can establish harmonic vibrations or wave motions so severe as to produce unacceptable product.
It is known to employ bridles to control the tensile load in strip moving through a continuous annealing furnace in order to maintain proper tracking and operating conditions within the furnace. Care must be taken, however, not to cause elongation of the strip in the high temperature section where the tensile load generally must be lower than is desired through the coating section of the line. It has been known, for example from U.S. Pat. No. 4,358,093, to provide bridles in the annealing furnace to isolate the tensile load in various sections of the furnace. Such bridles, when employed, conventionally have been of the short coupled two- or three-roll type utilizing a single SCR controller for all the bridle roll motor and drive mechanisms of each bridle. Such bridles installed in an annealing furnace substantially increase the overall size of the furnace, however, and may not be practical where space is a primary concern. In existing continuous annealing furnaces and hot dip coating installations, expanding an existing continuous annealing furnace to install a bridle in the final cooling zone will generally require moving the entire furnace since the hot dip coating pot and the cooling tower section of the building cannot readily be moved. It has been estimated that installing a conventional two-roll bridle in a continuous annealing oven of the type shown, for example, in U.S. Pat. No. 3,622,140 would increase the length of the oven by at least twelve feet and modification to the furnace alone would require capital expenditure of more than $1,000,000.00.
It has also been found that conventional bridles employing a single SCR controller for controlling all of the bridle roll drive motors do not provide the degree of tension control desired for a commercial high speed hot dip metal strip coating operation. Accordingly, there exists a need for improving the control of the tensile load in and the stability of steel strip in a hot dip galvanizing line above the coating metal bath until the coating has solidified. It is therefore a primary object of the present invention to provide an improved annealing and coating apparatus for use in a hot dip galvanizing line which will result in a more stable strip moving through the line and produce a more uniform quality of coated product.
It is a further object of the invention to provide an improved broad based bridle in the final cooling stage of the annealing furnace to provide a more uniform tensile load in the strip being galvanized while avoiding any increase in the overall size of the furnace.